PHOTOELECTROCHEMICAL IMAGING OF THE ETCHING AND PASSIVATION OF SILICON IN AQUEOUS KOH

Anisotropic chemical etchants are used widely to fabricate silicon mic rostructures; however, the roles of surface heterogeneity and localise d chemical effects during the etch process are still unclear. This inv estigation has employed photoelectrochemical microscopy to resolve spa tially the reactivity of p-Si [100] and [111] during etching and passi vation in 2.0 M KOH. Potentiodynamic photocurrent measurements demonst rate that the interfaces follow the ideal Gartner response until an an odic oxide is formed. Examination of the potential-dependent transient photocurrent response indicates that formation of an oxide inhibits c harge carrier transfer and pins the Fermi level. Furthermore, the grow th and dissolution of the oxide are reversible over a time scale of mi nutes. In situ optical and subsequent atomic force microscopies show t hat during the etching process surface roughening occurs preferentiall y at particular sites, the density of which is greater at [100] orient ed samples, and corresponds to regions at which pyramids are formed. P hotoelectrochemical microscopy results provide spatial information on the reactivity of interfaces, Both the [111] and [100] orientations ex hibit responses which are attributed to lattice defects at the semicon ductor surface. These act as recombination centres but do not manifest themselves in the final etch morphology. Examination of the influence of the electrode potential and effect of pre-passivation indicates th at the oxide formed is uniform and does not exhibit pinholes as in flu oride-containing media. However, real-time photocurrent imaging of the oxide removal process resulted in quite different behaviour, and an a dditional photoimage contrast was observed which differed considerably between the two crystal orientations. The origin of this heterogeneit y is attributed to the different dissolution kinetics of the oxides fo rmed.